Apparatus for plasma or reactive ion etching and method of etching substrates having a low thermal conductivity

ABSTRACT

Disclosed is a vacuum reactor for etching substrates having a low thermal conductivity to a high degree of etch rate uniformity, wherein the substrates to be etched are arranged in a holder at a predetermined spacing from the cathode to which RF energy is applied. According to a preferred embodiment of the invention, the cathode is raised in the area of the substrate to be etched to within a spacing of about 0.2 mm from the bottom side of the substrate. The cathode is made of aluminium, and is provided in the area of the substrate to be etched with a layer which acts as a black radiator. The heat formed during RIE is removed by radiation, and the radiation reflected from the cathode to the substrate is absorbed by the layer. Also disclosed is a method of etching substrates having a low thermal conductivity, in particular plastic substrates.

This is a continuation of copending application Ser. No. 07/422,928filed on Oct. 17, 1989 now abandoned.

DESCRIPTION Field of the Invention

This invention relates to an apparatus for plasma or reactive ionetching (RIE) and to a method of etching substrates having a low thermalconductivity, in particular plastic substrates, in such apparatus.

Background of the Invention

In the past few years, the etching of semiconductor substrates,dielectric materials and plastic substrates has been marked by a shiftfrom wet chemical to what is known as dry or plasma etch processes. Such"dry" processes, i.e. processes carried out in a vacuum, which are usedprimarily in semiconductor technology but also in the processing ofplastic materials, require an accurate temperature control. Thus, forexample, etch processes, such as PE (plasma etching), RIE (reactive ionetching), ECR (electron cyclotron resonance plasma etching), MRIE(magnetic confinement reactive ion etching), triode etching, CAIBE(chemically assisted ion beam etching), photon assisted etching, and thelike, necessitate cooling to prevent damage to the substrates to beetched or the films, such as photoresist films, arranged thereon.

Therefore, substrate holders are generally heated to a moderatetemperature by scavenging them with liquids, such as water or specialoils. Heat transfer between the substrate and the substrate holder is,however, impeded by the gap existing therebetween, which has led to anumber of suggestions as to how this might be remedied. U.S. Pat. No.4,282,924, for example, describes an apparatus for ion implantation,wherein the substrates are pressed against the convexly curved surfaceof a substrate holder by means of a ring or clamps, and heat transferbetween the substrates and the substrate holder, through which thecooling liquid flows, is improved by a thin intermediate layer of anelastomer. U.S. Pat. No. 4,399,016 describes a substrate holder, whereincontact to the substrate is improved by the latter being fixedlyattached by electrostatic attraction between the substrate and thesubstrate holder electrically insulated therefrom. Heat transfer canalso be improved by filling the gap between the substrate and thesubstrate holder with a metal (U.S. Pat. No. 4,129,881).

During vacuum treatment, an article is best cooled by introducing a gaswith a high thermal conductivity between the article to be treated, suchas, a semiconductor wafer, and the substrate holder. Suitable gases arenitrogen, neon, hydrogen and, in particular, helium (U.S. Pat. No.4,514,636 and many others). Effective heat transfer between thesemiconductor wafer and the substrate holder is ensured only if a staticgas pressure ranging from about 1.33 to 13.3 mbar, preferably of about1.33 mbar, is maintained between the two elements. This, in turn,requires that the substrates to be treated be mechanically orelectrostatically fixed to the liquid-cooled substrate holder and besealed from the reaction chamber to prevent an ingress ofheat-transferring gas which might noticeably affect the productionresult.

The previously-described apparatus and methods require mechanicallystable substrates which may be readily clamped and sealed from thereaction chamber. In any of these cases, heat transfer is effected byheat conductance in the material or by convection in gas, with theuniformity of cooling depending respectively on the support of thesubstrate on the substrate holder and the homogeneity and pressure ofthe heat-conducting gas. Only in a few special cases will the entiresubstrate surface rest on the substrate holder. Normally, the supportwill be a multi-point support which poses no problems where the etchrate, as, for example, during plasma or reactive ion etching, isindependent of temperature and/or where the thermal conductivity of thesubstrates to be etched is very high, also ensuring a uniform substratetemperature.

There are, however, substrate materials whose etch rate is stronglytemperature-dependent. This holds in particular for plastic substrates.Plastic materials frequently have a low thermal conductivity so thathighly irregular etch rates may occur if the substrates are fixed andcooled as described in the previously-cited prior art. Thus, forinstance, etch rate differences of more than a factor of two have beendetected within a few square centimeters of substrate surface during thereactive ion etching of substrates of polyoxymethylene homo- orcopolymers, leading to poor yield.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to provide an apparatus forplasma or reactive ion etching substrates having a low thermalconductivity, in particular plastic substrates, wherein the substratesmay be heated and treated uniformly across their entire surface

Now, there is provided an improvement in a vacuum reactor for etching asubstrate having a low thermal conductivity to a high degree of etchrate uniformity, comprising a process chamber, means for introducing aprocess gas into the process chamber, means for connecting a vacuum pumpto the process chamber, an electrode mounted in the process chamber, acounterelectrode in the process chamber, a substrate holder associatedwith the electrode for supporting a substrate to be etched and an energysource for applying RF energy to one of the electrode or thecounterelectrode. According to the invention, the substrate holder isarranged at a predetermined spacing from the electrode and the electrodeis provided with a radiation-absorbing layer.

The invention also provides an improved method of etching a substratehaving a low thermal conductivity in a vacuum reactor as described aboveat a pressure ranging from about 100 to about 10⁻³ mbar by electricallyor optically energized particles, using a process gas supplying theparticles. According to this aspect of the invention, the methodinvolves maintaining the substrate at a predetermined distance from theelectrode surface and removing the heat formed during etching byradiation.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of the preferred embodiments of the invention, asillustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an apparatus for treating a substrate in a reactive ionetch process;

FIG. 2 shows part of a cathode arrangement according to FIG. 1;

FIG. 3 shows part of another cathode arrangement according to FIG. 1;and

FIG. 4 shows an enlarged cross-sectional view of a throughhole in aplastic substrate after reactive ion etching.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

During the plasma or reactive ion etch process, the substrates to betreated are positioned on the lower temperature-controlled electrode ofa parallel plate reactor which is normally grounded during plasmaetching and to which a voltage is applied during reactive ion etching.The pressures used are between about 100 and about 10⁻³ mbar, with thepressures employed for reactive ion etching ranging from about 10⁻² toabout 10⁻³ mbar. The respective pressure is controlled by the capacityof the vacuum pump and the total flow of the process gas which istypically between about 3 and about 300 sccm. The process gas iselectrically or optically energized. Electrical energization may beeffected by a low-temperature plasma discharge which is produced bysingle-phase D.C. or two-phase A.C., whereas optical energization may beeffected, for example, by laser beams or ultraviolet light. As theenergized particles, such as ions, radicals, atoms or molecules impingeupon the substrates to be treated, a sizeable energy flux is inducedinto the substrates, causing the latter to be heated which, aspreviously described, must be checked by controlling the temperature ofthe substrate holder.

The method according to the invention will be described with referenceto the production of arbitrarily shaped micromechanical components ofplane-parallel plates of polymer material or arbitrarily shapedthroughholes produced therein by reactive ion etching. This processforms the subject-matter of European Patent application 87104580.3. Theplane-parallel plates of polyoxymethylene homo- or copolymers comprisingcircular and elongated holes produced by the process described in thatapplication are preferably used in a wire guide concept of test headsfor multilayer ceramic substrates. Test heads of this type are coveredby the subject-matter of European Patent application 87104577.9.

The process described in European Patent application 87104580.3 providesfor throughholes 41 (FIG. 4) to be produced by reactive ion etching inabout 1 mm thick polyoxymethylene plates (commercially available underthe designation "Delrin" from E. I. duPont de Nemours) having a diameterof about 2 to about 5 cm. For this purpose, the substrates 44 which arebilaterally covered with photoresist masks 42 with apertures 43 areintroduced into an RIE system, such as Leybold-Heraeus VZK 550 A. Thethroughholes 41 are reactive ion etched into the Delrin substrate 44,first from the front side and then, after the sample has been turned,from the back side down to a depth of about 2/3 of the substratethickness. As the two etch zones overlap in the center region,high-quality throughholes, i.e., strictly vertical openings withexcellent positional tolerance of the entry and exit holes on the frontand back sides, may be etched.

RIE is preferably effected in an oxygen plasma containing, if desired,up to about 50% argon, at a gas flow of about 5 to about 100 sccm, apressure range of about 1 to about 50 μbar and a single-phase D.C. biasof about 200 to about 900 V for a time sufficient to obtain the desiredetch depth. The parameters of the above-described RIE system are asfollows:

    ______________________________________                                        pressure:             10 μbar                                              gas flow:             30 sccm oxygen,                                                               12 sccm argon                                           HF amplitude:        900 V                                                    single-phase D.C. bias:                                                                            890 V                                                    substrate temperature:                                                                              70° C.                                           etch rate:          1.25 μm/min.                                           ______________________________________                                    

As previously mentioned, plastic materials generally have a low thermalconductivity. The plastic material, Delrin, for instance, has a thermalconductivity of about 0.3 Watt/m×°C. and a thermal coefficient ofexpansion of about 110×10⁻⁶ per °C. A silicon wafer by comparison has athermal conductivity of about 157 Watt/m×°C. and a thermal coefficientof expansion of about 2.33×10⁻⁶ per °C. In addition to its low thermalconductivity, the etch rate of Delrin is strongly temperature-dependent.Therefore, it is essential that the substrates are effectively cooled toensure that they are uniformly heated to a moderate temperature duringRIE. However, the Delrin substrates may not be fixed to the cathode asdescribed in the above-mentioned references, as, owing to the highcoefficient of expansion of Delrin, the planarity of the Delrinsubstrates, in comparison with a silicon wafer, for example, would nolonger be ensured.

To eliminate the problems of substrate cooling by heat conductance forsubstrates with a low thermal conductivity or plastic substrates, theinvention provides for the substrates to be fixed to a suitable cathodesuch that the resultant heat is substantially removed by radiation. Forthis purpose, the substrate must be arranged at a predetermined spacingabove the cathode and fixed without any tension. As the RIE process iscarried out within a pressure range of about 10³¹ 2 to about 10⁻³ mbar,heat removal by gas convection is largely negligible, since cooling bygas convection only becomes effective at a gas pressure of about 1.33mbar. See U.S. Pat. No. 4,514,636 and G. Fortuno et al., "ElectrostaticWafer Holder For Wafer Cooling During Reactive Ion Etching", IBM Tech.Discl. Bull., Vol. 31, No. 1 June 1988), pp. 462-464.

FIGS. 1 to 3 show, respectively, an apparatus and two cathodes withholders in which the substrates are positioned at a predeterminedspacing above the cathode during RIE. FIG. 1 shows an RIE system 1 witha process chamber 2, which is designed as a vacuum recipient and whichcomprises a connector 3 to a vacuum pump and an inlet 4 for introducinga process gas. Chamber 2 also comprises an energy source 5 from whichthe cathode 6 receives electric energy. The back side of the cathode 6is surrounded by a grounded screen 8, and the wall of the chamber 2 isformed by the grounded anode 7. The substrates to be etched 9 arearranged in a holder at a predetermined spacing above the cathode 6.Relevant details are shown in FIGS. 2 and 3.

FIG. 2 shows in a two-piece ceramic frame 25 and 26 a Delrin substrate23 to be etched bilaterally. The ceramic frame is forced against thecathode by spacers 24 having a low thermal conductivity. To permit theDelrin substrate to expand during heating, hollow spaces 27 are providedon either side of the holders. The cathode 21 is made of aluminium andis provided on its surface with a black radiator 22, such as an anodizedlayer. The black anodized surface is particularly advantageous, since itacts as an absorber and is resistant during RIE. The spacing of thesubstrate from the cathode is about 0.5 mm in the illustratedarrangement.

In all tests, the substrate temperature is measured by a pyrometer.

If the substrate, as provided in the art, is arranged to directlycontact the cathode, then a particular plasma potential, such as about660 V RF, and removal of the heat produced in the substrate byconductance to the cathode will produce a substrate temperature of about40° C.

In a further test, the substrate is positioned at a spacing of about 3mm to "float" above a cathode, whose surface is not provided with ablack radiator. At the same plasma potential, the final temperature ofthe substrate is about 93° C. This is due to the fact that the substrateis practically supported only on two quartz spacers which permit onlyvery little heat to be conducted to the cathode, with most of theplasma-produced heat in the substrate being removed by radiation. Thenoticeably higher substrate temperature in this arrangement may be adisadvantage which has to be accepted as a trade-off for an arrangementin which the substrates are directly supported on the cathode. A greatadvantage of such an arrangement is that for substrates with a lowcharacteristic thermal conductivity, a much more uniform temperaturedistribution and, thus, an improved uniformity of the etch rate isobtained.

Measurements have shown that with the above-described substrate holder,the heat radiated from the substrate in the direction of the aluminiumcathode is largely reflected by the aluminium surface, so that, as aresult, the substrate is additionally heated. By coating the aluminiumsurface with a black radiator or by anodizing such surface (FIG. 2),this radiation may be absorbed, lowering the substrate temperature fromabout 93° C. to about 70° C. The spacing of the substrate from thecathode is about 0.5 mm. The cathode arrangement depicted in FIG. 2 isable to achieve a very favorable uniform temperature and etch rate. Itmay be desirable, however, to also improve the electrical uniformity,i.e., during RIE care must be taken so that the ions are perpendicularlyincident on the substrate.

An undisturbed perpendicular incidence of the ions on the cathodesurface during RIE is ensured only with a totally planar cathode surfaceof infinite extension. Any geometrical irregularity of the cathodesurface will lead to a warping of the equipotential surfaces, so thatthey are no longer parallel to the substrate surface, and, since thethree-dimensional course of the equipotential surfaces determines theion paths, to a disturbed perpendicular incidence of the ions. Thearrangement of FIG. 2, for example, acts as a non-planar dielectric in aplate capacitor which is formed by the edge of the dark space and thecathode surface. It can be proven by experiment that the etch structureslocated close to the edges of the ceramic holders 25 and 26 are etchedobliquely. This is negligible if the etch structures are not too deep.If, on the other hand, their depth is great or if throughholes are to beetched, for example, into Delrin substrates (FIG. 4), obliquely etchedstructures are no longer tolerable.

The result of the experiment can be confirmed by computation.Computation was based on a spacing of 0.2 mm between the ceramic and thecathode surface, a thickness of 0.8 mm for the ceramic holding lips anda thickness of 1 mm for the Delrin substrates. The dielectric constantsused were ε=2.9 for Delrin and ε=9 for ceramic. The size of the darkspace during RIE was 15 mm and the D.C. potential was 500 V. Computationshowed that an ion incident on a 1 mm thick substrate in a substrateholder (FIG. 2) at a spacing of about 4 mm from the ceramic edge isinclined by about 1.5° to the vertical, which at a substrate thicknessof 1 mm corresponds to a stagger of the holes on the front and backsides of 28 92 μm.

The effect of oblique etching can be reduced by the arrangementillustrated in FIG. 3. FIG. 3 depicts in a two-piece ceramic frame 35and 36 a Delrin substrate 33 to be etched bilaterally. The ceramic frameis forced against the cathode by spacers 34 having a low thermalconductivity.

To permit the Delrin substrate to expand during heating, hollow spaces37 are provided on either side of the holders. Cathode 31 is made ofaluminium. It differs from the arrangement of FIG. 2 in having amodified cathode surface. In the area of the substrate to be etched 33,cathode 31 is raised in the form of a pedestal to within a spacing ofabout 0.2 mm from the bottom side of the substrate. In the area of thesubstrate, the cathode surface is provided with a black radiator 32, inthis preferred embodiment with an anodized layer. This particular designof the cathode, which ensures that the ions are perpendicularlyincident, in particular in the peripheral region of the substrateholder, reduces errors resulting from oblique etching by about 50 percent. The results of the experiment were also confirmed by computation.

With the aid of the cathode/holding means arrangements, (FIGS. 2 and 3),a wide variety of substrates having a low thermal conductivity and whichare sensitive to temperature during RIE can be etched highly uniformly.With regard to electrical uniformity, the arrangement of FIG. 3 isparticularly advantageous. The high degree of etch rate uniformitypermits a noticeable improvement in the yield of etch processes, inparticular those used to etch wire guide plates of Delrin.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A vacuum reactor for etching a substrate to ahigh degree of etch rate uniformity, said substrate having a low thermalconductivity and being sensitive to temperature, said vacuum reactorcomprising a process chamber, means for introducing a process gas intosaid process chamber, means for connecting a vacuum pump to said processchamber, an electrode, having upper and lower surfaces, mounted in saidprocess chamber, a counterelectrode in said process chamber, a substrateholder associated with said electrode for supporting a substrate to beetched and an energy source for applying RF energy to said electrode orsaid counterelectrode, wherein said substrate holder is arranged at apredetermined spacing above the upper surface of said electrode, saidpredetermined spacing ranging from about 0.2 mm to about 3 mm from theupper surface of said electrode to allow substrate cooling by heatremoval by radiation during etching, and wherein the upper surface ofsaid electrode, which surface is opposite said substrate holder, isprovided with a radiation-absorbing layer to remove by radiation heatformed during etching.
 2. The vacuum reactor of claim 1, wherein, duringreactive ion etching, said electrode, with which said substrate holderis associated, is the cathode to which said energy source applies RFenergy, and said counterelectrode is the grounded anode, and duringplasma etching, said electrode is the grounded anode, and said energysource applies RF energy to said counterelectrode.
 3. The vacuum reactorof claim 1, wherein an upper surface of said substrate holder extendsparallel to the surface of said electrode.
 4. The vacuum reactor ofclaim 1, wherein said substrate holder comprises two pieces and is madeof a ceramic material.
 5. The vacuum reactor of claim 4, wherein saidholder has a hollow space to allow the substrate to expand as it becomesheated during etching.
 6. The vacuum reactor of claim 1, wherein saidsubstrate holder is forced against said electrode by a spacer.
 7. Thevacuum reactor of claim 1, wherein said electrode is made of aluminumand has an upper surface which is coated with a radiation-absorbinglayer which acts as a black radiator.
 8. The vacuum reactor of claim 7,wherein an upper surface of said aluminum electrode is provided with aradiation-absorbing layer which is a black anodized layer.
 9. The vacuumreactor of claim 1, wherein in the area of the substrate to be etched,said electrode is raised in the form of a pedestal to within a spacingof about 0.2 mm from the bottom side of the substrate, the upper surfaceof said electrode, which surface is opposite said substrate holder,being provided with a layer which acts as a black radiator in the areaof the substrate.